One of the most striking examples of a "trophic" dependence of the olfactory bulb on the epithelium is the downregulation of dopamine (DA) in bulb juxtaglomerular neurons following deafferentation or nares occlusion. Work in the last funding period extended this phenomenon of "transneuronal regulation of transmitter phenotype" to two new peptides (CCK & CRF) and two new classes of neurons (other tufted cells & mitral cells), thus indicating that it is a general phenomenon. In the last funding period we discovered that the olfactory receptor neurons (ORNs) are both afferent to and the target of the DA cells which act via DA D2 receptors on the terminals of ORNs. This leads to the hypothesis that DA presynaptically regulates olfactory nerve terminals. Preliminary electrophysiological data support this hypothesis. Electrophysiological experiments using a newly developed rat olfactory bulb slice preparation and in vivo recordings will test the novel hypothesis that the DA neurons function to presynaptically regulate the excitability of olfactory nerve terminals. If this hypothesis is correct it will be important to learn how DA release is regulated. In vivo microdialysis experiments will test the hypothesis that activity in the olfactory nerve modulates the tonic release of DA. Previous studies suggest that the dependence of DA on the olfactory nerve is due either to [a] loss of an afferent trophic factor or [b] loss of afferent activity onto the DA cells. The finding that the DA cells "target" the ORN terminals suggests the hypothesis that target- derived trophic factors maintain the DA phenotype. This "target hypothesis" will be tested in a series of tissue culture and neural transplantation experIments. Nares occlusion experiments have been taken as evidence that afferent neural activity drives the DA phenotype. However, there is now strong evidence that increased neural activity causes a rapid increase in the expression of target derived trophic factors. Thus, reducing neural activity in the olfactory nerve by nasal occlusion could reduce target derived trophic factor. Tissue culture experiments with Pixley will be done to dissociate neural activity from trophic effects to determine which factor controls the DA phenotype. Together, these experiments will: [1] test novel hypotheses about the functions of one of the largest populations of neurons in the olfactory bulb, and [2] identify the mechanism(s) of transneuronal regulation of the DA phenotype; this will provide evidence for trophic functions of the olfactory nerve.